1. Field of the Invention
The present invention relates to a control system for hybrid vehicle, particularly to a control system for hybrid vehicle that reduces shock at clutch engagement when switchover from electric motor-powered driving to internal combustion engine-powered driving is instructed during electric motor-powered driving.
2. Description of the Related Art
There is known a hybrid vehicle equipped with an internal combustion engine, an electric motor connected to the output shaft of the engine through a clutch, and an automatic transmission connected to the output shaft of the motor. The automatic transmission is input with the output torque of the engine or the motor, or both the engine and the motor, and transmits the torque to driven wheels at a controlled speed (gear) ratio. During driving of such a hybrid vehicle under the power of the motor, if the driver presses down on the accelerator pedal in an attempt to accelerate, for example, the clutch engages to switch the source of driving power to the engine.
If the switchover is effected simply by starting the engine and engaging the clutch, however, the addition of the engine output torque to the motor output torque produces a sudden increase of the vehicle drive torque that may impart a shock to the passengers.
Japanese Laid-open Patent Application No. 2000-23312 teaches a technique for reducing this shock by providing a second motor to handle engine starting and using the second motor to absorb the output torque of the engine while the engine speed is being adjusted to match the motor speed.
More specifically, the method employed by this prior art technique is to detect the output torque of the second motor while it is absorbing the output torque of the engine, estimate the engine output torque, and adjust the desired torque of the motor connected to the automatic transmission based on the difference between the detected torque and the estimated torque.
Owing to the difficulty of accurately estimating the output torque of the engine, however, this known technology cannot always realize adequate shock absorption. As the torque transfer behavior of the clutch is hard to predict, moreover, this prior art technique is also less than satisfactory in control response to instantaneous torque changes.
A first object of the present invention is therefore to overcome these problems by providing a control system for hybrid vehicle equipped with an internal combustion engine, an electric motor connected to the output shaft of the engine through a clutch, and an automatic transmission connected to the output shaft of the motor that is input with the output torque of the engine or the motor, and transmits the torque to driven wheels, which can effectively reduce shock at clutch engagement during switchover from electric motor-powered driving to internal combustion engine-powered driving, when an instruction to switch to vehicle driving powered by the engine is made in a course of vehicle driving powered by the motor.
A second object of the present invention is, more specifically, to provide the system mentioned above which can effectively reduce the shock at clutch engagement during switchover from electric motor-powered driving to internal combustion engine-powered driving, when an instruction to switch to vehicle driving powered by the engine is indicated, for example, by the vehicle driver as an indication to accelerate in a course of vehicle driving powered by the motor.
The present invention achieves these objects by providing a system for controlling a hybrid vehicle equipped with an internal combustion engine, an electric motor connected to an output shaft of the engine through a hydraulic clutch and an automatic transmission connected to an output shaft of the motor to be inputted with an output torque outputted from at least one of the engine and the motor to transmit the output torque to driven wheels of the vehicle, comprising: initial control conducting means for conducting an initial control to supply priming hydraulic pressure to the clutch to take up dead stroke and to start the engine, when an instruction to switch to vehicle driving powered by the engine is made in a course of vehicle driving powered by the motor; speed difference determining means for determining a speed difference between speeds of the motor and the engine; increase control conducting means for conducting an increase control to increase the output torque of the motor and to increase supply of hydraulic pressure to the clutch based on the determined speed difference, when the engine is started; synchronous control conducting means for conducting a synchronous control to reduce the output torque of the engine for a predetermined period of time such that the engine speed is synchronized with the motor speed when the speed difference becomes less than a predetermined threshold value.